Others have proposed template matching techniques for more precisely controlling the size, positioning and number of picture elements ("pixels") that are printed on a xerographic photoreceptor to render bitmapped images.
Walsh et al., U.S. Pat. No. 4,437,122, describes a method of enhancing the resolution and quality of characters of a system receiving video display pixel information and providing hard copy output. The system accomplishes this by storing at least three successive lines of video data in successive parallel connected shift registers, applying the output of the shift registers to a decoder, and generating driving signals for a printer head. As described in column 2, lines 6-10, the decoder "compares the pixels on the same lines as well as in preceding and succeeding lines that surround each specific input pixel to generate the printer head driving signal according to whether straight or curved line segments are to be formed". As described in column 3, line 67, to column 4, line 1, the enhancement of the central pixel may be determined by "progressively examining an ordered table of matches to find an equivalent image and its related enhancement."
Tung, U.S. Pat. No. 4,847,641, describes a technique for enhancing the printing of bitmapped images by piecewise matching of the bitmap with predetermined stored templates of patterns to detect occurrence of preselected bitmap features. Templates representing compound error elements common to all bitmap images, associated compensation signals for each template, and the rules governing the relationships between the matched templates and the associated compensation signals are compiled into an index matching table implemented in a high speed parallel logic array.
Template matching effectively overcomes some of the sampling errors that are caused by the use of input data that is too coarse to accurately represent the higher spatial frequency content of the image. It does not, however, solve the problems that may be encountered in existing printers due to non-linearity in the way in which the spatial positioning of the transitions in printed images tracks changes in the intensity of the transitional boundary scans.
Many of the ROSs (raster output scanners) that have been developed for xerographic printing employ a single beam or a multi-beam laser light source for supplying one or more intensity modulated light beams, together with a scanner (such as polygon scanner) for cyclically deflecting the modulated laser beam or beams across a photoreceptor in a "fast scan direction" while the photoreceptor is being advanced simultaneously in an orthogonal "process direction." In practice, each of the laser beams typically is brought to focus on or near the photoreceptor surface to provide a substantially focused "scan spot." The scan spot, in turn, scans the photoreceptor in accordance with a predetermined scan pattern because the fast scan deflection of the laser beam or beams vectorially sums with the process direction motion of the photoreceptor. Indeed, the scan pattern is dependent upon and is determined by the scan rate (scan/sec) of the scanner, the spot size that is employed, and the process speed (inches/sec) of the photoreceptor. Such a scan pattern produces an exposure pattern because the scans are superpositioned on the photoreceptor, regardless of whether the scans simultaneously or sequentially expose the photoreceptor. Accordingly, it is to be understood that the present invention applies to printers and other display means that employ single beam or multi-beam ROSs, even though this disclosure features the single beam/single scan spot case for the sake of simplification.
Microaddressable printers and other types of display systems operate in an overscanned mode to render images by scanning one or more intensity modulated scan spots over a high gamma, photosensitive recording medium in accordance with a scan pattern that causes the spot or spots to superimpose multiple discrete exposures on the recording medium on centers that are separated by a pitch distance that is significantly less than the effective spatial diameter of the scan spot (i.e., the full width/half maximum (FWHM) diameter of a gaussian scan spot). Overscanned systems have substantially linear addressability responses, so boundary scans that are intensity modulated in accordance with the preselected offset values are used by these systems for spatially positioning the transitions that are contained by the images they render to a sub-pitch precision.
With older techniques of FWHM printing, the output resolution is limited to the scan resolution. With double overscanned printing, there are twice as many scans, effectively doubling the resolution scans in the process direction. The power to the laser diode, or "intensity" of each scan, can be controlled during the scan, giving subscan addressability. For example, in a printer with a spot size of 1/400 inch, scanning would take place at 800 scans per inch. At four levels of gray, the addressability of the printer increased to 3200 bits per inch.
Overscanning that results from the use of finer pitch scan patterns degrades the spatial frequency response of the printer in the process direction. A limited overscan is, however, consistent with the printing of high quality images because it permits the image transitions (i.e., the high spatial frequency content of the images) to be mapped onto the scan pattern with increased spatial precision.
The technique of microaddressability via overscanned illumination is more fully described in the copending, coassigned U.S. patent application Ser. No. 07/736,989 of D. N. Curry and D. L. Hecht, entitled "Microaddressability via overscanned Illumination for Optical Printers and the Like Having High Gamma Photosensitive Recording Media," incorporated herein by reference.